The ability of computer users to access programs and share data through local area networks (LANs) has become a readily expected part of most working environments. The improved efficiency within a particular LAN environment is often enhanced with the convenience of remotely accessing the LAN. An important extension to LANs is the provision of a wireless LAN (WLAN).
In a WLAN, end station units suitably employ radio communication using an FCC allotted frequency band of 2400 MHz (megaHertz) to fulfill performance expectations of wired LANs but without costly wire installation. One example of a WLAN is illustrated in FIG. 1. As shown, three end station units 10, 12, and 14 are in range with one another and have formed a portion of a WLAN 16. Also included in WLAN 16 is an access point station 18 that can access both connection oriented and connection-less services. Access point station 18 thus may support connection to both a local Ethernet backbone and some form of telecommunication transport, such as ISDN, ATM, or T1, as is well appreciated by those skilled in the art.
With the inclusion of connection oriented stations within a WLAN, the connection oriented services provide a potential mechanism for reserving bandwidth, such as for real-time or time-bounded data transfers, which may require a high data transfer rate. The connection-less services suitably provide typical asynchronous access to the medium in an fashion similar to Ethernet. Potential contention among varying unit types for communication in the WLAN raises significant issues for consideration in the development of standards for the WLAN. Included in these issues are how units will synchronize among themselves and how reserving bandwidth occurs.
In the prior art, attempts to solve LAN time synchronization problems include designations of one or more end station units/nodes as masters. In a single-master LAN, one node is designated as the master node and the clock of the designated node is utilized as the timing standard for every other node connected to the network. This requires the master node to periodically send a special time synchronization message to all other nodes within the network. Thus, all nodes in a single-master system must be capable of receiving messages from the master.
In general, on any wireless LAN, electrical noise and interference, coupled with signal attenuation effects corresponding to node separation distances, signal path obstacles, or multipath fading arising from signal reflections, preclude continuous reliable communication between a node and another specified node. Since each node within a single-master LAN system must be able to communicate with the master node, a single-master LAN system does not allow the flexibility in a wireless LAN environment.
Another problem associated with single-master LAN systems pertains to the master node recovering from a temporary communication problem. If a new master node has been selected during a time interval in which the original master node could not communicate, the LAN will have two master nodes after the original master node's recovery. Synchronization differences between the two master nodes results in conflicting synchronization being sent over the LAN.
In a multiple-master LAN, two or more nodes serve as masters for time synchronization purposes. Nodes within this type of LAN collect synchronization messages from each master at regular intervals, and typically calculate a corresponding average correct time. This requires each node to have the capability to communicate with at least one master node. As discussed above, in a wireless LAN environment, reliable communication between two specific nodes is not always possible. Thus, multiple-master systems are also undesirable for wireless LANs.
An attempt to provide time synchronization in a wireless LAN environment is described in U.S. Pat. No. 5,408,506. In this patent, each node is capable of transmitting and receiving information from at least one other node through frequency hopping spectrum communication. Time information is incorporated into all messages transferred between nodes by incorporating a value from the node's local clock into a message. A node is capable of receiving any message transmitted from other nodes within a predetermined receiving distance, regardless of whether or not the data in the message is addressed to the node. Upon message receipt, the receiving node stores the message header and the value of its local clock. The receiving node's virtual master clock processor then uses the time information contained within the message to calculate the time difference between the sending node's local clock and its local clock value. Prior to frequency hopping to another channel, a node's virtual master clock processor averages all determined time differences since the previous frequency hop, thereby creating a virtual master clock value corresponding to the average of the local clock values for all nodes from which messages were received. The virtual master clock processor then uses this average to adjust the node's local clock value to maintain synchronization.
While this method does produce synchronization within a wireless LAN, the need to store clock values from all the transmitted packets within a predetermined range is cumbersome. Further, tracking of the number of packets received to determine an average adds to processing overhead. Accordingly, a need exists for a time synchronization technique for a wireless LAN that is effective and efficient through utilization of more straightforward synchronized time value determination procedures.